Conventionally, in a head-up display device, a technique for reducing uneven brightness of an image, which is projected on a display screen such as a windshield, is well known. For example, a lighting device described in JP-B2-4437675 includes a liquid crystal display element for generating an original image of a projected image, a light emitting diode for emitting light toward the liquid crystal display element, and a concave mirror for projecting the original image on a display screen. A concave lens is arranged between the concave mirror and the liquid crystal display element, so that uneven brightness of the projected image is reduced, and further, a magnification ratio between the projected image and the original image increases.
The functions of the concave lens arranged between the concave mirror and the liquid crystal display element will be explained as follows. The intensity of light emitted from the liquid crystal display element is reduced when the angle of the light with respect to the optical axis directed from the light emitting diode to the concave mirror becomes large. Accordingly, when the light transmitted through the liquid crystal display element along with the optical axis is diffused by the effect of the concave lens, the brightness of light at the center of the projected image on the display screen is restricted. On the other hand, the brightness of light at the periphery of the projected image is increased. Thus, the uneven brightness of the projected image is reduced.
When the concave lens is positioned between the concave mirror and the liquid crystal display element such that the concave surface of the concave lens faces the concave mirror, the following difficulties may arise. The difficulties relate to ambient light, which is reflected on the projected image. Specifically, the ambient light enters from a certain direction, and is reflected by the concave mirror. Then, the reflected ambient light reaches the concave surface of the concave lens, which faces the concave mirror. The ambient light enters the concave mirror, and then, is reflected toward the concave mirror by a part of the concave surface, which is positioned perpendicularly to the optical axis directed from the light emitting diode to the concave mirror. The reflected ambient light together with the light image of the original image transmitted through the liquid crystal display element is projected on the display screen. Thus, the ambient light is reflected on the projected image on the display screen.